SQL Server error: Timeout expired. The timeout period elapsed prior to completion

Timeout expired. The timeout period elapsed prior to completion of the operation or the server is not responding.

This is a client-side timeout (ADO.NET, ODBC, your app's connection library) giving up after waiting longer than its configured limit — it almost always means the query itself is slow, not that the network or server is broken.

Common causes

• Missing indexes forcing a table/clustered index scan on a large table
• Blocking — another session is holding a lock the query needs, and it's queued behind it, not actually running slow itself
• Outdated statistics causing the optimizer to pick a bad execution plan
• Parameter sniffing — a cached plan optimized for a very different parameter value
• A default command timeout (often 30 seconds) that's simply too short for a legitimately heavy report/batch query

How to fix it

1. First determine if it's actually slow or blocked: check sys.dm_exec_requests / sys.dm_os_waiting_tasks for the session while it's running.
2. If it's a genuinely slow query, get the actual execution plan and look for scans, missing index suggestions, and high-cost operators.
3. If it's blocking, find the head blocker and address that session/transaction, not the one timing out.
4. Update statistics (UPDATE STATISTICS) if the plan looks stale relative to actual data volume.
5. Only raise the client timeout as a last resort — it hides the problem rather than fixing it.

Related

• Error 1205 — Deadlock victim